Anytime anyone says, "all it needs is, ... ", you can be sure it does not. If it was that simple, they would have done it so the car would be worth more when they sell it. They are selling it and did not do the repairs because of cost or difficulty.
The most common suspect is a leaking heater core, especially if there was red "Dex-Cool" antifreeze in the system. GM advertised that as "lifetime" coolant to make their cost of regular maintenance appear to be lower than that of their competitors, but then they put a sticker on the reservoir that says to replace it every three years. Even the Dex-Cool company never recommended waiting that long. The additives in it wear out in about two years, just like with any other antifreeze. Once that happens, it is normal for acids to form in the system that attack the metal parts. GM owners are very familiar with leaking radiators and heater cores. That can happen to any brand of car, but not with one as new as yours.
To verify my suspicion, look under the hood on the passenger side of the firewall for a four inch long rubber hose with a ninety degree bend hanging down. That is the drain tube for the water that condenses out of the air when the AC system is running. You should see only clear water dripping from that hose. It will form a puddle just behind and to the left of the right front tire. If you see coolant leaking from that hose, replace the heater core.
Heater hoses also are in that area. A hose could be dry-rotted and leaking or a clamp could be loose or rusted off. Be careful when tugging on those hoses. Most GM heater cores have plastic nipples that snap off easily if you pull too hard on a hose.
Thursday, June 29th, 2017 AT 4:10 PM